1. Technical Field
The present invention relates to an electronic balance of an electromagnetic force balancing type for measuring a load by balancing the load and an electromagnetic force applied to a balance beam by interposing a fulcrum. More particularly, the present invention relates to an electronic balance having a short distance from fulcra or an electronic balance with high accuracy.
2. Related Art
Generally, an electronic balance of an electromagnetic force balancing type is constituted such that an electromagnetic force generated by an electromagnetic force generator is operated to a balance beam engaged with a measuring pan, and the balance beam is balanced against a load operated to the measuring pan. A displacement of the balance beam is detected by a position (displacement) sensor and an amount of current supplied to a coil of the electromagnetic force generator is controlled by a servo mechanism to make a result of detecting the displacement from a base point position of the balance beam null. A magnitude of the load operated to the measuring pan, that is, a measured weight value (hereinafter, abbreviated as measured value) is calculated from the current amount required for balancing.
Here, a variation in the measured value caused by vibration of the balance beam in accordance with a disturbance vibration is preferably symmetrical on both sides of a value in correspondence with a balance point of the balance beam. Therefore, a position of balancing the balance beam, that is, an attitude of the balance beam to nullify the result of detecting the displacement by the position sensor is generally set to be horizontal. That is, the balance beam is pivotably supported centering on the fulcrum, a load point of the load to be measured operated in a vertical (gravitational force) direction and an operating point of the electromagnetic force similarly operated in the vertical direction are provided at positions respectively remote from the fulcrumby inherent distances. However, the more deviated the attitude of the balance beam from a horizontal-direction, the smaller the torque operated on the balance beam by the forces centering on the fulcrum.
When the balance beam is set to be brought into a balanced state by the horizontal attitude, the balance beam is vibrated centering on the horizontal attitude. Therefore, the current flowing in the coil is varied symmetrically centering on a current amount in correspondence with the balance point in a state of vibrating the balance beam, and an average value thereof coincides with the current amount in correspondence with the balance point.
Therefore, by setting the balance beam to bring about the balanced state by the horizontal attitude, an accurate measured value can be displayed even under an environment of a large disturbance vibration. Thus, the electronic balance of the related-art is provided with a position sensor to nullify the result of detecting a displacement in the horizontal state (See JP-A-10-104045).
Although the electronic balance of the related-art is constituted as described above, there is a case in which the balance beam of the electronic balance is displaced in a direction other than the gravitational force direction by transportation or an ageing change of the electronic balance or an external vibration. When such a situation is brought about, sensitivity and linearity of the electronic balance is changed from those in adjusting the electronic balance and in accordance therewith, high accuracy is deteriorated or measurement at high speed and with high stability cannot be carried out by external vibration. There poses a problem that particularly in the case of an electronic balance having a short distance from fulcra or a highly sensitive electronic balance, an influence thereby is considerable.